Adhesion process

ABSTRACT

A process of bonding two or more articles together using an adhesive composition comprising at least one polyhydroxyalkanoate (PHA) wherein the PHA is placed between the articles to be bonded and the articles are subjected to pressure to set the adhesive, optionally at elevated temperatures.

This application claims benefit of international applicationPCT/GB94/01496, filed Jul. 11, 1994.

THIS INVENTION relates to an adhesion process. More particularly itrelates to a process of bonding articles together with an adhesivecomprising a biodegradable polymer.

It is known to coat paper with polyhydroxyalkanoic acids (PHA), forexample polymers of hydroxybutyric acid (PHB) and copolymers withhydroxyvaleric acid (PHBV) both as latex and as dry solid, preferably asdry powder. The coating is biodegradable but is non-adhesive.

We have now found that articles may be stuck together using PHA as anadhesive under cold application conditions and setting it with botharticles in contact with one another under pressure and preferably withheat. The adhesive is biodegradable. The adhesive is believed to set bycrystallisation.

Thus, according to the present invention there is provided a process ofbonding two or more articles together using an adhesive compositioncomprising at least one polyhydroxyalkanoate (PHA) wherein the PHA isplaced between the articles to be bonded and the articles are subjectedto pressure to set the adhesive, optionally at elevated temperature.

The PHA is especially one capable of a relatively high level ofcrystallinity, for example over 30%, especially 50-90%, in the absenceof plasticiser. Suitably it is or includes at least onemicrobiologically produced polyester having units of formula I:

    --O--C.sub.m H.sub.n --CO--                                I

where m is in the range 1-13 and n is 2 m or (if m is at least 2) 2 m-2.Typically C_(m) H_(n) contains 2-5 carbon atoms in the polymer chain andthe remainder (if any) in a side chain. In very suitable polyesters n is2 m and there are preferably units with m=3 and m=4 copolymerisedtogether and with respectively a C₁ and C₂ side chain on the carbon nextto oxygen in the chain. The polymer may be homopolymer, especially apolyhydroxybutyrate (PHB), or a copolymer, especially a copolymer ofhydroxybutyric acid and hydroxyvaleric acid (HV) containing preferably4-30 mol % of m=4 units. Thus, particular polyesters contain apreponderance of m=3 units, especially with 70-98 mol % of such units,the balance (if any) being units in which m=4. The molecular weight ofthe PHA is preferably over 50000, especially over 100000, up to e.g.2×10⁶.

The PHA may be a blend of two or more copolymers differing in the valueof m. A particular example contains

(a) PHA consisting essentially of Formula I units in which 2-5 mol % ofunits have m=4, the rest m=3; and

(b) PHA consisting essentially of Formula I units in which 5-30 mol % ofunits have m=4, the rest m=3.

In each such PHA there are preferably side chains as above mentioned.The proportions in such a blend are preferably such as give an averagem=4 content in the range 4-30, more preferably 10-28, mol %.

In each such PHA having units with m=3 and m=4 there may be very small,typically fractional, percentages of units having higher values of m.

Typically the composition contains microbiologically produced PHA to theextent of over 50, especially over 80,% w/w.

The PHA is preferably a fermentation product, especially of amicrobiological process in which a microorganism lays down PHA duringnormal growth or is caused to do so by cultivation in the absence of oneor more nutrients necessary for cell multiplication. The microorganismmay be wild or mutated or may have had the necessary genetic materialintroduced into it. Alternatively the necessary genetic material may beharboured by a eukariote, to effect the microbiological process.

Examples of suitable microbiological processes are the following:

for Formula I material with m=3 or m=partly 3, partly 4: EP-A-69497(Alcaligenes eutrophus);

for Formula I material with m=3: U.S. Pat. No. 4,101,533 (A. eutrophusH-16), EP-A-144017 (A. latus);

for Formula I material with m=7-13: EP-A-0392687 (various Pseudomonassp.).

The PHA can be extracted from the fermentation product cells by means ofan organic solvent, or the cellular protein material may be decomposedleaving small granules of polymer for example of which less than 10%have diameters greater than 1 micrometres. For specialised end uses thecellular protein may be partly or wholly allowed to remain with the PHA,but preferably subjected to cell breakage.

Alternately, the PHB or PHBV can be a product of synthetic chemistry(Bloembergen and Holden, Macromolecules 1989, 22, p1656-1663.Bloembergen, Holden, Bluhm, Hamer and Marchessault, Macromolecules 1989,22, p1663-1669).

The PHA may contain one or more nucleants, for example, boron nitride.

The PHA may optionally contain a variety of additives making it suitablefor a range of applications. Such additives include tackifiers, waxes,plasticisers, stabilizers or antioxidants and compatible thermoplasticpolymers. It is an advantage of the invention that good results can beobtained in the absence of tackifier.

The adhesive composition preferably contains 20 to 99% by weight of PHBor the copolymer PHBV, particularly 50 to 95%, especially 50 to 90%.

If a tackifier is present, the composition preferably contains 5 to 80%w/w thereof. Examples of suitable tackifiers include rosin and rosinderivatives, terpene phenolics and pure phenolic resins; and morespecifically, gum rosin, wood rosin, tall oil rosin, distilled rosin,hydrogenated rosin, dimerized rosin, polymerised rosin, glycerol-, orpentaerythritol- esters of natural and modified rosins, such as forexample wood rosin, the glycerol- or pentaerythritol- ester ofhydrogenated rosin, the phenolic-modified pentaerythritol ester ofrosin, phenolic modified terpene resins and hydrogenated derivativesthereof, and thermoplastic alkyl phenolic resins such as those describedin U.S. Pat. No. 4,073,776 and U.S. Pat. No. 4,023,826.

A combination of two or more of the above mentioned tackifers may beused if suitable.

If a wax diluent is present, the composition preferably comprises 1 to30% w/w thereof. Suitable examples are 12-hydroxystearamide wax,hydrogenated castor oil, oxidized synthetic waxes and polyethyleneoxide.

If a stabilizer is present the composition preferably contains 0.1-4%w/w thereof. Suitable examples are high molecular weight phenols andmultifunctional phenols such as sulphur and phosphorus-containingphenols, for example,1,3,5-trimethyl-2,4,6-tris-(3,5-di-tert-butyl-4-hydroxy-benzyl)benzene;pentaerythritoltetrakis-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate;n-octadecyl-3,5-di-tert-butyl-4-hydroxyphenol)-propionate;4,4'-methylenebis (2,6-tert-butylphenol); 4,4'-thiobis(6-tert-butyl-o-cresol); 2,6-di-tert-butylphenol;6-(4-hydroxyphenoxy)-2,4-bis(n-octyl-thio)-1,3,5-triazine;di-n-octadecyl- 3,5 di-tert-butyl-4-hydroxybenzylphosphonate;2-(n-octyl-thio) ethyl-3,5-di-tert-butyl-4-hydroxy-benzoate; andsorbitol hexa 3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate.

If a plasticiser is present, the ratio thereof to PHA may be up to 40phr w/w and the range 5-20 especially 6-12, phr w/w is generallysuitable.

Examples of suitable plasticisers are high-boiling esters of polybasicacids, such as phthalates, isophthalates, citrates, fumarates,glutarates, phosphates or phosphites. The esterified radicals may be forexample C₁ -C₁₂ alkyl, aryl, aralkyl or aralkyl. Particular examples aredialkyl, for example dioctyl, phthalates, and dialkylalkylene oxideglutarates high-boiling esters and partial esters of polyhydricalcohols, especially glycols, polyglycols and glycerol. The acid-derivedradical of the ester typically contains 2-10 carbon atoms. Examples aretriacetin, diacetin and glycerol dibenzoate and aromatic sulphonamidessuch as paratoluenesulphonamide

Particular examples of such plasticisers are esters of polyhydricalcohols, for example glyceryl esters of C₁ -C₄ carboxylic acids,especially Estaflex (Tradename). Generally it is preferred that theplasticiser should be biodegradable.

Additional optional additives include colorants such as titaniumdioxide, and fillers such as talc and clay. These may be present insmall amounts usually 0-20%, preferably 5 to 20%.

Particularly preferred PHA adhesive compositions comprise 50-95% PHA,0-20% of plasticiser, 0-1% of nucleant, 0-20% of tackifier, 0-4% ofstabiliser or antioxidant, 0-10% of wax diluent, and 0-20% of a colorantand/or filler.

PHA adhesive formulations can be, for example, granules, dry powder orpreferably latex suspensions. In such formulations the PHA is inparticulate form before the setting treatment. The adhesive layer may beat least partly discontinuous after the setting treatment. Granules maybe formed by a normal extrusion process, using for example, a single ortwin screw extruder in which the PHA powder is melted and passed througha die before cutting into granules.

Amorphous PHA tends to crystallise with time and it may be supplied asan aqueous suspension or latex, in which the average particle size isfrom 0.1 μm to 2.0 μm and preferably from 1.0 μm to 2.0 μm.

Suitable latices can be produced in known processes for producing PHA,particularly polymers of hydroxybutyric acid and copolymers thereof withhydroxyvaleric acid, in which microorganisms are grown under conditionssuch that they accumulate PHA and are then treated to break theircellular structure and to release particles of PHA into an aqueoussuspension. This can be accomplished for example by heat treatmentand/or digestion with enzymes. Organic matter other than PHA ispreferably largely removed, for example by separating the PHA from theliquid, preferably in the presence of a surfactant.

The PHA may be resuspended in water to provide a suitable latex for useas an adhesive. Suitable latices and methods of their production aredescribed in European Patent 145,233. Those containing peptidoglycan asthe major non-PHA cell material may be used. The peptidoglycan isbelieved to be present as a layer surrounding the substantiallyamorphous PHA particles. It need not be completely removed as it tendsto isolate the particles from surrounding materials which may inducecrystallisation. The latex may be stabilised with a surfactant.

The process of the present invention may be carried out without heatingif a latex is used. If powder or granules are used heating is usuallynecessary. The adhesive composition is placed between the two articlesin the cold state, preferably at a temperature from 15° C. to 50° C.,especially 15° C. to 35° C. Then the articles are subjected to pressureand optionally a heat treatment to set the adhesive. Typically thepressure is applied in the range 500 to 2000 and up to 10000 psig (34.5to 690 bar), especially 500 to 8000 psig (34.5 to 550 bar) for settingat the "cold" temperatures mentioned above. Typically the heat treatmentif used is in the range 120 to 190, especially 130° to 190,° C.:pressures up to 2000 psig (138 bar) are then usually sufficient.

The adhesives of the present invention are useful for a wide range ofapplications. Of particular interest are sealing operations for example,in packaging and carton sealing, in sanitary napkins, disposable babynapkins and hospital equipment, etc. The adhesives are useful fordisposable articles in which at least one flexible film material isbonded to at least one tissue, non-woven, polyolefin or other flexiblepolymeric film substrate. Alternately, the adhesive may bond elastic topolyethylene, polypropylene or a non-woven substrate.

The adhesive composition when set does not necessarily have to provide apermanent or strong bond in order to satisfy the requirements of aparticular bonded article. In some cases it may be advantageous for thebond to be weak enough to enable the articles to be peeled apart. Theadhesive layer contains PHA typically at 0.1 to 10.0 g/dm².

Latex suspensions may be used in the absence of nucleants, tackifiers,stabilisers, antioxidants, plasticisers, colorants and fillers ifdesired.

The following examples illustrate the invention but do not in anywaylimit the scope of the invention.

Latex suspension preparation

A sample of fermentation broth from a culture producing PHBV (11% molHV) was heat shocked to break open the cells and treated with aproteolytic enzyme followed by a water wash regime as described inEP-A-145 233. At this stage in the preparation a sample of latexsuspension was taken for use in the present invention. The molecularweight of the PHBV in the latex suspension was 527,000.

Granule preparation

PHBV dry powder (24% mol HV) prepared by methods known in the art(available from ZENECA Bioproducts, PO Box 2, Belasis Avenue,Billingham, Cleveland, TS23 1YN, UK) was used to prepare PHBV granules(24% mol HV) containing 10 phr triacetin and 1 phr boron nitride. Thegranules were prepared by extrusion at a die temperature of 155° C.(maximum). The molecular weight of the PHBV in the granules was 294,500.

Materials Specification

Paper=76.02 g/m²

Aluminium Foil=33.81 g/m²

PHB-Film=72.94 g/m²

Card=340 to 550 g/m²

Sample preparation Latex suspension adhesive (no heat treatment)

Strips (10 cm×2 cm) of a range of materials were cut. Latex solution(40% w/w of PHA; 1 ml) was pipetted on to the first test material. Asecond test material was overlaid on the first with a 5 cm overlap andthen the combined test sample was put between two pieces of MELINEX(trade mark) film and placed between two plates in a press at roomtemperature (20° C.). The plates were adjusted until they touched thetest sample and then pressure was applied to 5 tonnes (490 bar) and heldfor 1 minute. The pressure was released and the sample removed from theplates. The samples were cut into 2 cm wide strips.

Latex suspension adhesive--(heat treatment)

Strips of a range of materials to be tested were cut into 20 cm×9 cmstrips. Latex suspension (1 ml) was pipetted on to an area in the centreof the first strip of test material with a 5 cm overlap. The secondstrip of test material was overlaid on top of the first strip of testmaterial. Then the combined test sample was put between two pieces of"MELINEX" (trade mark) film and placed between two plates in a press.The press temperature was set to either 190° C. or 134° C. Pressure wasapplied to 5 tomes (109 bar) and held for 1 minute. The sample wasremoved quickly and allowed to cool. When completely cold the sampleswere cut into 2 cm wide strips.

Granule adhesive--(heat treatment)

Strips (20 cm×9 cm) of the materials to be tested were cut. One piece oftest material was laid on a press with "MELINEX" (trade mark) filmbetween it and the plate. Between 1.5 g and 1.8 g of granules wereplaced in the intended 5 cm overlap area and the second test materialwas laid on top of the first test material. "MELINEX" (trade mark) filmwas placed between the second test material and the second plate. Theplates with the test sample between were transferred to a press at 190°C. or 134° C. The pressure was increased to 5 tomes (109 bar) and heldfor 1 minute at 190° C. or 45 seconds at 134° C. The sample was thenremoved from the press and cooled. When completely cool the samples werecut into 2 cm wide strips.

Peel test for adhesive strength

All the samples were tested for the strength of the adhesive bond usingan Instron 1122C mechanical test machine at a 90 degree peel angle,speed 10 mm per minute and at a temperature of 23° C. The bond strengthis presented according to the following scale:

    ______________________________________                                                        Gic (Nm.sup.-2)                                               ______________________________________                                        VERY POOR         less than 1                                                 POOR              between 1 and 10                                            LOW               between 10 and 100                                          GOOD              between 100 and 500                                         VERY GOOD         over 500                                                    ______________________________________                                    

in which Gic (Nm⁻²) is (force÷width)×(1-cos Q) where Q is the peelangle. The results are presented in Table 1.

                  TABLE 1                                                         ______________________________________                                        SPECIMEN      ADHESIVE  TEMP      BOND                                        BASE    TOP       TYPE      °C.                                                                            STRENGTH                                  ______________________________________                                        PAPER   PAPER     LATEX      20     LOW                                       PAPER   PAPER     LATEX     190     GOOD                                      PAPER   Al-FOIL   LATEX      20     POOR                                      PAPER   Al-FOIL   LATEX     190     POOR                                      PAPER   PHB-FILM  LATEX     134     GOOD                                      CARD    PAPER     LATEX      20     POOR                                      CARD    PAPER     LATEX     190     GOOD                                      Al-FOIL Al-FOIL   LATEX     190     LOW                                       PHB-FILM                                                                              PHB-FILM  LATEX      20     POOR                                      PHB-FILM                                                                              PHB-FILM  LATEX     134     V GOOD                                    CARD    PHB-FILM  LATEX      20     POOR                                      CARD    PHB-FILM  LATEX     134     GOOD                                      CARD    CARD      LATEX      20     V POOR                                    CARD    CARD      LATEX     190     GOOD                                      PAPER   PAPER     GRANULE   190     GOOD                                      PAPER   PAPER     GRANULE   134     GOOD                                      PAPER   Al-FOIL   GRANULE   134     POOR                                      PAPER   PHB-FILM  GRANULE   134     GOOD                                      CARD    PAPER     GRANULE   134     GOOD                                      Al-FOIL Al-FOIL   GRANULE   190     POOR                                      Al-FOIL Al-FOIL   GRANULE   134     POOR                                      Al-FOIL PHB-FILM  GRANULE   134     POOR                                      CARD    Al-FOIL   GRANULE   134     POOR                                      CARD    PHB-FILM  GRANULE   134     GOOD                                      CARD    CARD      GRANULE   190     GOOD                                      CARD    CARD      GRANULE   134     GOOD                                      ______________________________________                                    

I claim:
 1. A process of bonding two or more articles together using anadhesive composition comprising at least one polyhydroxyalkanoate (PHA)wherein the PHA is applied to one or more of the articles as a latex ofparticles of PHA in water, placed between the articles to be bonded andthe articles are subjected to pressure to set the adhesive, said PHAbeing a polymer or copolymer of hydroxybutyric acid.
 2. A process ofbonding two or more articles together using an adhesive compositioncomprising at least one polyhydroxyalkanoate (PHA) wherein the PHA isapplied to one or more of the articles as a latex of particles of PHA inwater, placed between the articles to be bonded and the articles aresubjected to pressure to set the adhesive, said PHA comprising therecurring unit --O--C_(m) H_(n) --CO-- wherein m is 1-13 and n is 2 m.3. A process as claimed in claim 1 in which the PHA is a copolymer ofhydroxybutyric acid and hydroxyvaleric acid.
 4. A process as claimed inclaim 3 in which the content of hydroxyvaleric units in the copolymer isin the range 10 to 28 mol percent.
 5. A process as claimed in claim 1 inwhich the PHA is derived from a microorganism.
 6. A process as claimedin claim 1 which is carried out without heating and the pressure is inthe range 34.5% to 690 bar.
 7. A process as claimed in claim 1 in whicha nucleant, tackifier, plasticiser, antioxidant, stabiliser, colorant orfiller is present.